Digital data are recorded on CDs, DVDs, and other optical media by using a laser to create pits in the surface of the medium. The data can then be read by a laser moving across them and detecting variations in the reflectivity of the surface. While this method is effective for creating machine-readable features on the optical medium, those features are not easily legible to the human eye.
Materials that produce color change upon stimulation with energy such as light or heat may be used to create human-readable images. For ease of discussion, such materials will be referred to herein as “thermochromic materials” (which change color by the action of heat) and that term as used herein is intended to encompass photochromic materials (which change color by the action of light). Leuco dyes are one kind of thermochromic material and are particularly well-suited to use with optical media because they can be activated with the same laser that is used to burn digital data onto the optical media, with the result that a single system can be used to produce both machine- and human-readable data on a CD, DVD, or other optical device.
One type of thermochromic coating that can be used with a laser is an ink comprising a leuco dye, a proton source (developer), and an ink vehicle. In many cases the ink vehicle may be a mixture of radiation curable monomers and oligomers (UV-curable lacquer). The developer can be a proton source such as highly acidic phenol or any other suitable proton source.
Leuco dyes in their crystalline form have relatively low solubilities in the lacquer. By contrast, the amorphous forms of many leuco dyes have significantly higher solubilities. The developer often has good solubility in the lacquer. Thus, during ink preparation: a) developer is dissolved in the lacquer and forms a relatively stable solution; and b) leuco dye in the amorphous form is dissolved in the lacquer and allowed to crystallize into its less soluble crystalline form. The resulting ink typically consists of 2 distinctive phases: 1) crystallized leuco dye; 2) lacquer phase with developer dissolved in it. Alternatively, pre-crystallized leuco dye may be added to the lacquer.
Ink formulated this way may be printed/coated as a thin coating (1-20 um) and cured into polymer matrix by electromagnetic radiation (typically UV). A color change in the ink coating can be brought about by raising its temperature. Upon heating, at least one phase of the coating melts and the dye molecules begin to come into contact with developer. Intimate contact of leuco dye and developer at high temperature results in proton transfer from developer to leuco dye and causes a color change of the latter. Rapid cooling of the system preserves the color change by preventing re-crystallization of the dye. Because the melted area is relatively small, the coating is relatively thin, and the coating is in contact with the significantly thicker substrate, sufficiently rapid cooling is not difficult to achieve.
Because the dye becomes visible only when it has been melted and dissolved in the matrix, and because rate of the color development is highly dependent on the leuco dye dissolution rate, smaller crystallite sizes of the leuco dye translate into faster dissolution and color formation rate. Thus, the size of the leuco dye crystals greatly affects the imaging sensitivity of the ink. If the crystals are too large, the available laser power will not be sufficient to bring about a satisfactory color change fast enough, resulting in diminished marking sensitivity. In addition, larger crystals result in increased light scattering, which reduces efficiency of imaging laser energy absorption as well as legibility of the desired marks.
Because the crystals that occur naturally if when a leuco dye is crystallize out of solution are much larger than is desirable, it is often necessary to add the dye in the form of pre-formed crystals that have been milled. Milling the crystals to achieve the desired particle size increases the cost and complexity of the ink-making process. Hence, it is desirable to provide a method for using leuco dyes that avoids these deficiencies of known systems.